Spreader stoker fired furnace



NV- 2 1954 c, 1 MARQUEZ, JR

sPREAnER sToxER FIRED FURNACE 2 .Sheets-Sheet 1 Filed Aug.A 27, 1949Nov. 2, 1954 c. L MARQUEZ. JR

SPREADER STOKER FIRED FURNACE 2 Sheets-Sheet 2 Filed Aug. 27, 1949INVENTOR CizarlesL/I//arguegJ/f ATTORNEY United States Patent SPREADERSTOKERFIRED FURNACE Charles L.y Marquez, Jr., Jersey City, N. J.,assignor to The yBabcock & Wilcox Company, New York, N. Y., v 1acorporation `of New Jersey Application August 27, 1949, serial No.112,802 is Claims. (cl. 122-4) larly to theA construction andoperationrof spreader Stoker fired furnaces having, aforwardlyjmovingcontinuous grate and adapted for use in a vapor generating `unit.

' Spreader stokersv have come into use for firing a wide range of solidfuels, ranging kfrom lignites to the high grades of bituminous coals,`withlittle or no fuel preparation, screenings andv crushed run-of-minecoal being usable without pulverization. The fuel is mechanically thrownrearwardly from a fuel hopper arranged along the front furnace wall byone or more bladed rotors. In the usual projected stream of. fuel, asubstantial portion, e. g. 30%,4 burns yas fine particles held insuspension above the grate, while 'thecoarser particles deposit on thegrate at separated points and are burned thereon. Small openings in thegrate elements permit the passage of air for combustion at relativelylow pressure over the entire grate areal from an air box below thegrate. The portiony of 'the furnace above the grate must be leftunrestricted 'to afford sufficient furnace volume and height toaccommodate 'the desired fuel trajectories and combustion space for thefines and volatiles. The heat release rate of the fuel burned insuspension is quite high and heat reflecting or reradiating arches ofthe type heretofore used with Amoving grate fuel beds are neithernecessary norv desirable. When the grate has a multiplicity oflinkedsectional elements defining an endless chain, thegrate elements normallymove forwardly very slowly and continuously discharge the residual ashbelow the front wall into a suitable ash hopper.

With the described construction and arrangement of the fuel burningvpartsythe zone of most intense cornbustion isvnormally located in therear portion of the furnace opposite the wall of .fuel introduction. Incommercial practice, it hasbeen found difficult to operate furnaces ofthe type described under smokeless cornbustion conditions throughout'thedesired load range, and particularly at low operating loads. The smokeconditions 'are mainly due to therpresence in the gaseous products ofcombustion of unconsumed volatiles released by the fines wholly orpartly burned in suspension, particularly in the zone of most intensecombustion, and carried out in the gases under relatively stratifiedflow conditions. Under suchv flow conditions, the further mixingrequired between the volatiles and the air for complete combustion isdifficult to attain. For this reason it is customary to increase theair-fuel ratio by providing a greater percentage lof excess air in theupper portion of the furnace, to thereby complete combustion of theunburned fuelparticles and volatiles. An alternative method is tosodilute the stack gases with air that the unconsumed carbonconstituents of the gases are not so evident. In addition,-to improvecombustion conditions, overfire air has been introduced through portsinthe furnace walls. 'When such ports are also used -for the return offines, Asuch as cinders and fiy ash from other portions lofthe unit, tothe furnace, part of the fines so Suspended in the air jets will bepicked up and added vto the relatively'large amount of solids carryoverin the stack gases which is characteristic of spreader stoker firedfurnaces.

While a forwardly moving endless grate has special advantages when usedwith a spreader type stoker, a controlled distribution of the combustionair supply from the air-box is difficult unless the air boxcan becorrespond- 2,693,173 Patented Nov. 2, 1954 ingly subdivided', astructural arrangement difficult` with an 'endless chain grateconstruction. Accordingly, -it has been found 'that a substantialportion of the combustion air passingupwardly, 'particularly through theforward portion of the grate, is ineffective in the combustion processin ,that it tends to flow upwardly without anyv substantial mixing with'the products of combustion of the fuelpon the grate and the fuelparticles burned, in suspension. Also, the lack of any localized airflow. control results in the volatiles released in the zone of .intensecombustion receiving insufficient air Sto complete their combustion. f

` In accordance with my invention, a furnace designed for use with aspreader type vStoker arranged along its front wall 4and deliveringsolid fuel to afforwardly traveling grate, is provided with a specialfluid cooledtrea-r wall construction consisting of a lower vertical.section an upper inclined Wall portion projecting forwardly andupwardly to a substantial extent above the rear or high combustionsection of the grate and at a .level a :substantial height above thegrate fuel bed. For the purposes of the invention, this rear wall.should project in,- wardly at least twenty-five per cent (25%.) of thefurnace chamberlength at the grate level. 'The furnace `arclji, or roof`section so formed effects a progressive and 'substantial reduction inthe furnace cross-sectional flow area. at a level intermediate vtheheight of the furnace and has been found to materially assist inpromoting mixingand turbulence in 'the adjacent Apart of the furnace, bydiverting the products lof combustiony and the included incompletelyburned volatiles and carbon vrising from the Vzone of intense combustionat the rear section of the grate forwardly into the restrictedcross-sectional area of the furnace. This lateral flow of .hot gases notonly improves the internal mixing of the deflected l,gas stream, .butalso,

substantially increases the mixing of the defiected Igas stream with thehot gases passing upwardly through the forward .section of the grate, tocomplete the combustion of l'any unburned volatiles and `carbon fin themixed .gas streams before the gases reach the convection heated tubularheat transfer surface. Due to the increased infixing and turbulence ofthe gases effected, spreader Stoker fired furnaces with thisconstruction lcan '.be operated with lowerexcess air values and morecomplete `combustion conditions, whereby the smoke density will .beconsiderably reduced, or a wider .fange .of operation can be attainedwith the present smoke conditions. 'Thedesired Acombustion conditionsare 'thus attainable without recourse Yto overfire air jets located in.the furnace side walls. 'This arch construction substantially'increases the heating gas flow in 'and through the upper vportion o'fthe furnace, and a more efiicien't utilization of the-'heat transfersurface in the section 'thus results. The lower carryover of solids inthe `gases correspondingly decreases thecarbon loss, as well as tubeabrasion and fouling.

In the drawings:

Fig. 1 is a sectional elevation of a spreader stoker iirtd steamgenerating unit embodying the invention; an

Fig. 2 is an enlarged horizontal section taken on -the line 2--2 of Fig.l.

The .invention is illustrated as incorporated in a steam generatingyunit in which the setting enclo'ses a vertically elongated furnacechamber 10 along the front side 4of a generating and superheatingsection comprising an yupper steam and water ldrum 1'1, a lower waterdrum 12, and spaced groups of water tubes 13, 14 and' 15 connecting thedrums, the tubes 13 constituting a 'vertical bank of generatingl tubes,the tubes 14 constituting a bank of downcomer tubes spaced rearwardly ofthe tube bank 13, and the tubes 15 constituting two rows o'f screentubes of U-formation extending forwardly of the tube bank.

upper end of the Vtube bank 14 pass downwardly over economizer sections21 to an induced draft fan 22 from which they are discharged to thestack.

.Thefurnace of the unit is of rectangular horizontal cross-section anddefined by a watercool'ed front wall 25, side walls 26, rear wall 27,and roof 28. The front wall 25 and roof 28 -are lined with water tubes29 extending between the header and steam and water drum 11. The sidewalls 26 are lined with water tubes 31 extending from a bottom header 32vertically and then laterally `into a circumferential row in theV drum11. The rear furnace wall 27 is lined with a row of water tubes 35extending from a bottom header 36 upwardly to the drum 11,r ashereinafter described. Y

'The fuel burning equipment for the steam generating unit illustratedconsists of a spreader stoker'40, in cornbination with a forwardlymoving endless chain grate 45. The spreader stoker 40 is of any suitabletype for feeding solid fuel from the hopper 41 to side-byside bladedrotors 42 which distribute the fuel substantially uniformly over thewidth and length of the grate, through ports 43 located in the frontwall 25 below the refractory wall section 25a. The chain grate is formedin a series of side-by-side endless chain sections, each having drivingsprockets 46 at its rear end connected to any suitable drive mechanism(not shown), and idler sprockets 47 at its forward end, the forward endof the grate being arranged to discharge into an ash hopper 50 below thefront wall of the furnace chamber. The chain comprises link elementscarrying a series of perforated grate bars for supporting the fuel bedand permitting the flow of air therethrough. The lower part of the grateis enclosed in an air box arranged to receive a controllable supply ofair from a forced draft fan 56.

With this fuel burning equipment the solid fuel particles aremechanically discharged by the stoker over the length and width of thegrate area in the furnace chamber along the trajectories indicated inFig. 1. In normal operation, the chain grate is moved slowly forward,continuously receiving fresh fuel from the Stoker which burns during thesubsequent grate movement with the ash being discharged at the forwardend of the grate. The fine fuel particles discharged by the spreaderstoker are wholly or partly burned in suspension and facilitate rapidburning of the fuel particles depositing on the grate. With the grateconstruction shown, the combustion air from the air box will bedischarged at a relatively low pressure throughout the length and widthof the grate area. With such an arrangement the zone of most intensecombustion will be on and over the rear section of the grate, andusually with a deficiency in the amount of air required for completecombustion of the carbon and the volatiles driven olf from the fuel bedand fuel burned in suspension. Substantially all of the fuel in the fuelbed will be consumed by the time the corresponding grate section reachesthe forward end of the grate, and normally there will be a surplusamount of combustion air passing upwardly through this section.

ln accordance with my invention the rear wall of the furnace has aspecial configuration and construction for improving combustionconditions in the furnace and reducing solids carryover in the furnacegases. For this purpose the rear wall 27 has a lower vertical section27a extending to a height above the'normal maximum fuel trajectory andan upper forwardly inclined arch section 27lo at an angle ofapproximately 45 to the horizontal. In the embodiment illustrated, thearch 27b extends inwardly a distance equal to approximately 40% of thefurnace chamber length at the grate level. Thev inner and upper end ofthe arch section 27h is connected with the lower end of the inclinedbaille 19 and also to a horizontally arranged bale 20 formed by tileassociated with the screen tubes 15. The wall sections 271- and.27b arelined with bare water tubes 35 on their inner sides, the tubes 35 havingthe same configuration as the `wall sections, and extend upwardlytherealong to the juncture of the wall section 27b and baille 19. Abovethat level alternatetubes 35 are bent out of alignment to form two rowsof widely spaced slag screen tubes in front of the screen tubes l5. Theupper ends of the tubes 35 are bent into alignment and connected to thedrum 11. It will thus be noted that the longitudinal dimension of thefurnace chamber 10 is progressively reduced a substantial extent fromthe upper end of the vertical wall section 27% to approximately thelevel of the lower water 'drum 12. The

inclined water tube lined rear archthus formed by the wall section 27bis spaced sufficiently above the fuel bed to allow ample height for thefuel trajectories and combustion space for the lines and volatilesburning in suspension. The water cooled construction employed makes thearch a heat absorbing surface relative to the fuel bed rather than aheat reradiating surface intended to reflect heat to the fuel bed on thegrate.

As previously described, the inclined'arch 27b deilects the combustiongases and suspended fuel andash particles towards the front furnace wall25promoting turbulence and mixing of the gases in all restricted furnacearea. Complete combustion of the volatiles and carbon content of thesuspendedV particles -is thus effected `before the mixed gas streamturns laterally intocontact with the screen tubes 35 and 15and remainingconvection heated surface. The resulting increased gas flow through theupper section of the furnace substantially increases the heat absorptionrate of the furnace wall tubes .in that section with aconsequentincrease in thermal efliciency.

While in accordance with the provisions of the statutes l haveillustrated and described herein the best form of the invention nowknown to me, those skilled in the art will understand that changes maybe made in the form ofthe apparatus disclosed without departing from thespirit ofthe invention covered by my claims, and that certain featuresof my invention may sometimes be used to advantage without acorresponding use of other features. n

What is claimed Yis:

l. .ln a fluid heater, vertical front and side walls partly defining avertically elongated furnace chamber having a heating gas outlet in theupper rear portion thereof, a spreader Stoker atthe lower end of` saidfront furnace wall having means for mechanically projecting solid fuelin rearwardly directed trajectories in said furnace chamber, acontinuous grate extending` across the bottom of said furnace chamberand having 'substantially horizontal forwar ly moving upper grateportions arranged to receive on rear portions of said grate the fuelparticles projected by saidistoker, said fuel projecting means beingupwardly spaced from said upper grate portions, a combustion air boxbelow said grate and opening thereto throughout the grate area, meansfor supplying air to said air box, a rear wall for said furnace chambercomprising a lower vertical section extending from adjacent the gratelevel to an elevation abovesaid fuel projecting means of said stoker,said lower rear wall section being positioned directly over a portion ofsaid grate rearwardlyl of said fuel receiving portions, and an inclinedarch section confined to a lower portion of `said chamber` and extendingforwardly and upwardly over said-,rear portions of said grate andarranged toy deflect the gases rising from said rear grate portionsforwardly in said furnace chamber, said lower vertical rear wall sectionextending to. an elevation higherthan the summit of the maximumtrajectory of fuel projected by said Stoker.

2. in a fluid heater, vertical front and side walls partly defining avertically elongatedl furnace -chamberhaving a heating gas outlet in anupper portion thereof, a spreader stoker at the lower end ot' said frontfurnace wall having means for mechanically projecting solid fuel inrearwardly directed trajectories in `said vfurnace chamber, a continuoustraveling grateextending across the bottom of said furnace chamber andhaving substantially horizontal upper run portions arranged to receiveonA rear portions of said grate the fuel particles projected by saidStoker, said upper run portions being `downwardly spaced throughout fromsaid fuel projecting means, means for'continuously moving thefuelreceiving portions of said'grate forwardly in said furnace chamber, acombustion air box below said grate and opening thereto throughoutthearea of said fuel receiving portions, means for supplying' air` tosaidv air box,'a rear wall for saidufurnace chamber comprising@ lowervertical section extending from adjacent the grate level to an elevationabove said fuel projecting means of said Stoker, said lower rear wallsection being positioned directly over a portion of said graterearwardly ofy said fuel receiving portionsand an finclinedarchsectionconned to a lower portion of said chamber ,and extending forwardly overthe rear section ofsaid grate and arranged to deflect the gasesrisingjfrom saidrear grate section for; wardly in said furnace chamber,said lower vertica'lrear wallsection `extendingfto anelevationsufficiently.v above saidv fuel projecting means sov as to position saidarch section at an elevation higher than the summit of the maximumtrajectory of fuel projected by said stoker.

3. In a fluid heater, vertical front and side walls partly defining avertically elongated furnace chamber having a heating gas outlet in anupper rear portion thereof, a spreader stoker at the lower end of saidfront furnace wall having means for mechanically projecting solid fuelin rearwardly directed trajectories in said furnace chamber, acontinuous traveling grate extending across the bottom of said furnacechamber and having substantially horizontal upper run portions arrangedto receive on rear portions of said grate the fuel particles projectedby said stoker, means for continuously moving the fuel receivingportions of said grate forwardly in said furnace chamber, said upper runportions being downwardly spaced throughout from said fuel projectingmeans, a combustion air box below said grate and admitting air theretoat a common pressure throughoutlthe area of said fuel receivingportions, means for supplying air to said air box, a rear wall for saidfurnace chamber comprising a lower vertical section extending fromadjacent the grate level to an elevation above said fuel projectingmeans of said stoker, said lower rear wall section being positioneddirectly over a portion of said grate rearwardly of said fuel receivingportions, an inclined fluid cooled arch section confined to a lowerportion of said chamber and extending forwardly over the rear section ofsaid grate and arranged to deflect the gases rising from said rear gratesection forwardly in said furnace chamber, said lower vertical rear wallsection extending to an elevation sufliciently above said fuelprojecting means so as to position said arch section at an elevationhigher than the summit oi' the maximum trajectory of fuel projected bysaid stoker.

4. In a fluid heater, upright front, rear and side walls partly dening avertically elongated furnace chamber having a heating gas outlet in anupper rear portion thereof, a spreader stoker at the lower end of saidfront furnace wall having means for mechanically projecting solid fuelin rearwardly directed trajectories in said furnace chamber, acontinuous traveling grate extending across the bottom of said furnacechamber and arranged to receive on rear portions of said grate the fuelpartielM projected by said stoker, means for continuously moving thefuel receiving portions of said grate forwardly in said furnace chamber,a combustion air box below said grate and admitting air thereto at acommon pressure throughout the area of said fuel receiving portions,means for supplying air to said air box, said grate being arranged todischarge from front portions thereof the ash resulting from combustionof said particles, said rear wall for said furnace chamber comprising alower vertical section extending from adjacent the grate level to anelevation above said fuel projecting means of said stoker, an upper'inclined arch section confined to a lower portion of said` chamber andextending forwardly and upwardly over said rear portions of said gratefor a distance at least 25% of the furnace length at the grate level andarranged to deect the gases rising from lsaid rear grate portionsforwardly in said furnace chamber, said lower vertical rear wall sectionextending to an elevation suiciently above Said fuel projecting means soas to position said upper arch section at an elevation higher than thesummit of the maximum trajectory of fuel projected by said stoker, saidlower rear wall section having its lower end portion positioned directlyover a portion of said grate rearwardly of said fuel receiving portions.

5. In a stoker fired furnace in which fuel is burned in a forwardly andhorizontally movable bed, the method of operating said furnace whichcomprises, supplying fuel to the bed in rearwardly directed trajectoriesoriginating at a point upwardly spaced from the front end of the bed,directing air upwardly through the bed throughout the entire areathereof so as to cause the line fuel particles to be at least partiallyburned in suspension and thus facilitate rapid combustion of fuelparticles depositing on the bed whereby the zone of most intensecombustion is created in and above the rear portion of the fuel bedwhile the deposited fuel particles are progressively consumed during theforward movement of the bed and ash is continuously discharged from thefront end thereof, the progressive combustion of fuel as the bed movesfor wardly rendering the forward portion of the bed more permeable toair flow therethrough than the rearward portion and resulting in a ow ofexcess air through the forward portion and in a ow of air through therearward portion less than is required to support complete combustion ofunconsumed volatiles in the zone of most intense combustion; and, in alower part of said chamber, superjacent the summit of the maximumtrajectory of the rearwardly projected fuel particles, deflecting thegases rising from said rearward portion of the bed together with gasesresulting from the combustion of fuel particles in suspension so as todirect such gases forwardly and upwardly into the stream of gases andexcess air rising from the forward portion of the bed.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,823,050 Kerr Sept. l5, 1931 1,864,366 Murray June 21, 19322,110,452 Moyer Mar. S, 1938 2,333,631 Bailey et al. Nov. 9, 19432,347,574 Mosshart Apr. 25, 1944 2,416,053 Grossman Feb. 18, 19472,476,567 Sparks July 19, 1949 2,511,935 Midgette et al June 20, 1950FORElGN PATENTS Number Country Date 369,925 Great Britain Mar. 2l, 1932

